Many analog signal measurement applications present unique challenges, but high-accuracy field applications can be especially difficult. Environmental conditions, particularly temperature, have known effects on electronic components, but these effects can be costly to prevent or correct, especially across multiple components in combination. The dynamic range of signals that must be supported may impose additional constraints, particularly where personal safety is an issue.
One exemplary field application is that of electric service metering accuracy verification in the electric utilities industry. Metering installations are frequently outdoors and located in areas as diverse as the sub-arctic and the tropics necessitating stability over a large temperature range such as −20 to 70° C. Peak input voltages are commonly as high as 1000 VAC and, for the application of determining burden across a current transformer, may be as low as 10's of milliVolts (0.01 VAC). This demanding 5 decade input range is difficult to support, especially as the low range values need to be acceptably accurate, and particularly in an easily portable device. Regulatory requirements (ANSI, PUC) typically require better than 0.05% (500 ppm) accuracy in scenarios verifying a 0.2% field device, and may require 0.02% (200 ppm) or better accuracy as advances are made in field delivery technology.
A typical historical approach to cover large input signal ranges has been to create multiple overlapping input ranges with multiple gain stages to accommodate the most common subsets of the overall input signal range, and then switch measurement circuitry between ranges, either manually or automatically, as the signal varies.
Another approach using modern electronic circuits is to utilize a large register ADC (24-bit) to cover the entire range, but on a 1000 VAC range this can result in a single bit difference affecting the low level measurements by roughly 0.3% (at 20 mV), which is an unacceptably large proportion of the desired measurement accuracy. Additionally, modern ADC devices generally have pipelined architectures, and switching ranges typically causes a loss of data acquisition continuity. This can cause issues in applications like electric power measurement where acquisition must be continuous for time-normalized accuracy.
Mechanisms for minimizing and/or correcting the effects of environmental factors have historically been expensive, bulky, or both. They may also be expensive to apply in time, as when complex factory calibration procedures are used to periodically quantify and apply corrections. They may further be unacceptable due to subtle manufacturing details or scarce critical parts with long lead times.
These and other problems lead to various design goals that may be particularly desirable in a portable field measurement device: small physical size; ease of use; an optimum combination of high measurement accuracy over temperature and time; minimal energy consumption (i.e., long battery life); robustness in the field; low cost to manufacture; temperature-independent calibration; as well as other similar factors that may arise during manufacture and/or implementation of such devices.
While various combinations of these problems have presented challenges that have inspired numerous conventional approaches, there still exists a desire for a way in which many measurement applications may be separated into application subranges which are a priori mutually exclusive, obviating the need for dynamic switching during acquisition, and in which certain aspects of manufacturing electronic components can be profitably utilized to good effect.